Nanomechanical tests, such as nanoindentation (see References 1, 2 and 3), nano-tensile testing (see Reference 4), and nano-scratch test (see Reference 5), for example, are modern characterization methods to quantitatively evaluate mechanical properties of a sample at nanoscale. To obtain accurate mechanical properties, the force and displacement of a testing probe must be precisely controlled and monitored. However, in some environments, such as vacuum environments, the testing probe can experience serious motion control problems due to extremely low air damping in vacuum environments. For example, in a high vacuum/low damping environment, there is an increase in the mechanical amplification at the resonance frequency and an increase in the overall system settling time. Additionally, the low damping environment present in a high vacuum affects mechanical behavior by allowing large disturbing vibrations that create an unstable divergence in the closed loop control during nanomechanical testing. To solve the problems inherent in performing nanomechanical testing or scanning probe microscopy in a high vacuum, increasing the system damping is highly desired.
Damping controls have been attempted with many different methods and instruments (see Reference 6-16). One method is referred to as Q-control (see Reference 12). Q-control is designed for nanoscale measurement instruments, especially for the intermittent contact mode utilized in scanning probe microscopy (see Reference 11). This Q-control is realized with an analog circuit and, although it can be used to modify the system damping, it has limitations when applied to a broad range of bandwidth. Utilizing the Q-control, the bandwidth of the phase shifting circuit should be adjusted to manipulate the mechanical quality factor (Q) of the system near a certain resonance frequency and, for different frequency regions; a different circuit bandwidth is required. In addition, this analog damping controller is implemented into testing instruments as an add-on device, resulting in a more complicated hardware configuration.
Another damping control algorithm implemented with atomic force microscopes employs a trigonometric lookup table (see References 13, 14, and 15). This damping control algorithm is also designed for intermittent contact topography scanning and dynamic force spectroscopy with an oscillating probe operating at a specific frequency. This digital damping control, however, only works when oscillating the probe at a certain frequency and cannot be implemented in order to increase the system damping in non-oscillating motion control. With such a control system, the typical maximum Q modification is about 5 times the unmodified Q value (see Reference 13).
However, such a performance level falls short for effectively damping down the vibrating device in nanomechanical testers in high vacuum. Mechanical testers used in high vacuum usually have a high Q value (e.g. 6,000), and it is recommended to decrease the Q value by about 1,000 times to be reasonably controllable.
One known feedback influenced Q-control system (see Reference 10) changes the damping characteristics of the system. This known Q-control system is based on modifying the test system in a closed loop control scheme and the damping is modified through the use of PID (proportional-integral-derivative) controller gains. While damping modification with PID control can increase the force sensitivity of the oscillating force probe by enhancing the system Q value, adjusting the Q value using the PID gains results in a dynamic system having a small phase margin for dynamic stability.
In short, reducing the system quality factor in nanomechanical testers in high vacuum is highly desirable because it can shorten the settling time and improve the stability in a closed loop control mode. Although reducing the system quality factor is important for high measurement accuracy, to date, no active damping controller is known to have been developed for nanomechanical testing in high vacuum.